In baseball, a slider is a breaking ball pitch that tails laterally and down through the batter's hitting zone; it is thrown with less speed than a fastball but greater than the pitcher's curveball.

The break on the pitch is shorter than that of the curveball, and the release technique is 'between' those of a curveball and a fastball. The slider is similar to the cutter, a fastball pitch, but is more of a breaking ball than the cutter. The slider is also known as a yakker or a snapper.

A Hall of Fame pitcher famous for his slider was lefty Steve Carlton. Right-handed pitcher David Cone was famous for his slider, which he was able to use many different ways, as was Bob Gibson of the Cardinals.

More Info: en.wikipedia.org